As a working method of cutting an internal gear which is a typo of gear, there is broaching in which a broach is used as a cutting tool in which blades are arranged in a saw shape (for example, PTL 1). This is internal work performed on a workpiece by installing, in a broaching machine as a working machine dedicated to broaching, a broach and the workplace as a piece to be cut, and pulling the broach with respect to the workpiece or pulling the workplace with respect to the fixed broach.
A broach is a bar-shaped bladed material in which a large number of cutting blades are arranged to be formed in a saw shape, and the cutting blades of the broach are arranged in dimensional order such that the height and the blade width of the blades gradually increase from one end side (the leading end of the cutting direction) toward the other end side (the trailing end of the cutting direction). One of the features of the broach is that various working processes can be combined, in a single cutting tool. For example, in a case where a broach having a roughing section for roughing of the workplace and a finishing section for finishing of the workpiece is used, in the broaching machine, roughing and finishing are completed only by pulling the broach with respect to the workpiece once such that the workpiece can be forced to an internal gear.
Since broaching is performed by only pulling the workpiece or the broach once, working speed from roughing to finishing of the workpiece is faster than that of other internal work. In addition, the finished dimensions of the workpiece by broaching become substantially the same as those of the final blades (finishing blades at the rearmost end of the cutting direction) of the broach used for working, and thus cutting work can be performed with high accuracy and the repetition accuracy of the cutting work is high.
As a type of broach, there is a helical broach. This is for cutting a workplace to a helical internal gear in which the tooth lead of the internal gear is inclined with respect to the axis of the gear. The helical broach and the workplace are installed on a broaching machine and the helical broach is pulled with respect to the workplace while being rotated, thereby forming the workplace to the helical internal gear.
An example of an existing helical broach is illustrated in FIG. 4, and a method of finishing a workplace in the existing helical broach is illustrated in FIG. 5.
As illustrated in FIG. 4, a helical broach 101 includes a roughing section 103 and a finishing section 104, and roughing blades (not illustrated) in the roughing section 103 and finishing blades 150 (FIG. 5) in the finishing section 104 are arranged to be inclined with respect to the axial direction of the helical broach 101.
In the roughing blades (not illustrated) in the roughing section 103 and the finishing blades 150 in the finishing section 104, a gear tooth helix angle α is set along the tooth lead direction of the helical internal gear to be formed. In order to enhance the working accuracy and the like, in the finishing blades 150 in the finishing section 104, a blade groove helix angle β may further be set.
In addition, the blade groove helix angle β is set to a direction that is not perpendicular to the direction of the gear tooth helix angle α in order to enhance the working accuracy and the like. Therefore, as illustrated in FIG. 5, one end portion 151 (upper left portion in FIG. 5) in the finishing blade 150 has an acute angle, and the other end portion 152 (lower left portion in FIG. 5) has an obtuse angle.
In addition, in order to enhance the working accuracy and the like, in the finishing section 104 of the helical broach 101, a single finishing blade 150 is set to abut and cut only one of tooth surfaces including a left tooth surface 170 in a work-piece W (one tooth surface along the tooth lead of the workpiece W) and a right tooth surface 180 (the other tooth surface along the tooth lead of the workpiece W). That is, the finishing blades 150 in the finishing section 104 of the helical broach 101 are formed to be divided into left tooth surface finishing blades 150a which cut only the left tooth surfaces 170 in the workplace W and right tooth surface finishing blades 150b which cut only the right tooth surfaces 180 in the workpiece W.
Therefore, the left tooth surface finishing blade 150a performs cutting by allowing an acute angle portion 151 to abut the left tooth surface 170 in the workpiece W, and the right tooth surface finishing blade 150b performs cutting by allowing an obtuse angle portion 152 to abut the right tooth surface 180 in the workpiece W.